U.S. patent number 6,709,770 [Application Number 10/203,259] was granted by the patent office on 2004-03-23 for steel sheet hot dip coated with zn-al-mg having high al content.
Invention is credited to Atsushi Ando, Atsushi Komatsu, Nobuhiko Yamaki.
United States Patent |
6,709,770 |
Komatsu , et al. |
March 23, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Steel sheet hot dip coated with Zn-Al-Mg having high Al content
Abstract
A high Al hot-dip Zn--Al--Mg plated steel sheet is obtained by
forming on a steel sheet surface a hot-dip plating layer
comprising, in mass %, Al: more than 10 to 22% and Mg: 1-5%, and,
optionally, Ti: 0.002-0.1%, B: 0.001-0.045% and Si: 0.005-0.5%. The
plating layer exhibits a metallic structure of [primary crystal Al
phase] mixed in a matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic
crystal structure]. Substantially no Zn.sub.11 Mg.sub.2 phase is
present in the metallic structure of the plating layer.
Inventors: |
Komatsu; Atsushi (Izumi-shi
Osaka 594-0074, JP), Yamaki; Nobuhiko (Takaishi-shi
Osaka 592-0013, JP), Ando; Atsushi (Toyonaka-shi
Osaka 561-0881, JP) |
Family
ID: |
18556936 |
Appl.
No.: |
10/203,259 |
Filed: |
August 7, 2002 |
PCT
Filed: |
February 06, 2001 |
PCT No.: |
PCT/JP01/00826 |
PCT
Pub. No.: |
WO01/59171 |
PCT
Pub. Date: |
August 16, 2001 |
Foreign Application Priority Data
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Feb 9, 2000 [JP] |
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2000-32317 |
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Current U.S.
Class: |
428/659;
428/939 |
Current CPC
Class: |
C23C
2/40 (20130101); C23C 2/12 (20130101); C23C
2/06 (20130101); Y10T 428/12799 (20150115); Y10S
428/939 (20130101) |
Current International
Class: |
C23C
2/04 (20060101); C23C 2/06 (20060101); C23C
2/12 (20060101); B32B 015/18 (); C23C 002/06 () |
Field of
Search: |
;428/659,939,653
;427/433 ;148/533,537 ;420/519 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0905270 |
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Mar 1999 |
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EP |
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64-8702 |
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Feb 1989 |
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JP |
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10-226865 |
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Aug 1998 |
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JP |
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10-306357 |
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Nov 1998 |
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JP |
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11-140615 |
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May 1999 |
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JP |
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11-279732 |
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Oct 1999 |
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JP |
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Other References
DJ. Blickwede "55% Al-Zn-Alloy-Coated Sheet Steel" (Oct.
1979)..
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Primary Examiner: Zimmerman; John J.
Attorney, Agent or Firm: McDermott, Will & Emery
Claims
What is claimed is:
1. A high Al hot-dip Zn--Al--Mg plated steel sheet obtained by
forming on a steel sheet surface a hot-dip Zn-base plating layer of
a composition comprising, in mass %, Al: more than 10 to 22%, Mg:
1-5%, Si: 0.005-0.5% and the balance of Zn and unavoidable
impurities, which plating layer exhibits a metallic structure of
primary crystal Al phase mixed in a matrix of Al/Zn/Zn.sub.2 Mg
ternary eutectic crystal structure.
2. A plated steel sheet according to claim 1, wherein substantially
no Zn.sub.11 Mg.sub.2 phase is present in the metallic structure of
the plating layer.
3. A high Al hot-dip Zn--Al--Mg plated steel sheet obtained by
forming on a steel sheet surface a hot-dip Zn-base plating layer of
a composition comprising, in mass %, Al: more than 10 to 22%, Mg:
1-5%, Ti: 0.002-0.1%, B: 0.001-0.045%, Si: 0.005-0.5% and the
balance of Zn and unavoidable impurities, which plating layer
exhibits a metallic structure of primary crystal Al phase mixed in
a matrix of Al/Zn/Zn.sub.2 Mg ternary eutectic crystal
structure.
4. A plated steel sheet according to claim 3, wherein substantially
no Zn.sub.11 Mg.sub.2 phase is present in the metallic structure of
the plating layer.
Description
TECHNICAL FIELD
This invention relates to a high Al hot-dip Zn--Al--Mg plated steel
sheet whose plating layer has an Al content on a level of more than
10 to 22 mass %.
BACKGROUND ART
The good corrosion resistance of hot-dip Zn--Al--Mg plated steel
sheets produced using a plating bath containing suitable amounts of
Al and Mg in Zn has long made them a focus of various development
and research. In the production of hot-dip plated steel sheet of
this type, however, spot-like crystal phase appears on the plated
steel sheet surface. After standing for a while, the spot portions
turn grayish black and give the sheet surface an ugly appearance.
Despite being excellent in corrosion resistance, therefore, hot-dip
Zn--Al--Mg plated steel sheet has been slow to gain acceptance as
an industrial product.
Through extensive studies the inventors ascertained that the
spot-like crystal phase is Zn.sub.11 Mg.sub.2 phase. Based on this
finding, they defined a metallic structure for a Zn--Al--Mg plating
layer containing Al: 4-10% and Mg: 1-4% that inhibits
crystallization of the Zn.sub.11 Mg.sub.2 phase and presents a good
appearance. They also developed a production method for obtaining
the metallic structure. The metallic structure and production
method are described in JPA. 10-226865 and JPA. 10-306357.
OBJECT OF THE INVENTION
Thanks to the metallic structure and production method proposed by
the inventors, it has become possible to produce industrial-quality
hot-dip Zn--Al--Mg plated steel sheet with a plating layer Al
content on the 4-10% level that does not have an ugly spotted
appearance. However, no study has been reported regarding whether
production of such a high-quality hot-dip Zn--Al--Mg plated steel
sheet is possible when the Al content is high, e.g., when the
plating layer contains Al in excess of 10 mass %. The literature
also offers little data regarding the corrosion resistance of
hot-dip Zn--Al--Mg plated steel sheet having an Al content in such
a high region.
On the other hand, it is known that increasing the Al content of a
Zn-base plating offers such advantages as improved heat resistance.
This suggest that it could well be worth while to look into the
feasibility of developing commercial Zn--Al--Mg plated steel sheet
products in the high Al region of an Al content exceeding 10 mass
%. In fact, however, little research has been done in this
direction.
The reason for this can be traced at least in part to the reported
corrosion resistance of Zn--Al plated steel sheet found in outdoor
exposure tests. These show that corrosion resistance improves with
increasing Al content up to a plating layer Al content of around 10
mass % but then begins to degenerate when the content exceeds about
10 mass %. It was held that the tendency to degenerate in corrosion
resistance would continue up to an Al content of approximately 20
mass % (See Iron and Steel, 1980, No. 7, p.821-834, FIG. 2). As
nothing contrary to this was reported, it came to be considered an
established theory. In including Al in a Zn-base plating layer,
therefore, the ordinary practice is, from the viewpoint of
corrosion resistance (particularly outdoor exposure performance),
to avoid the Al content range of approximately 10-20 mass %.
Moreover, when the Al content of the plating layer exceeds 10 mass
%, an alloy layer composed mainly of an intermetallic compound
between the steel sheet base metal and the plating layer very
readily forms. This, too, has hindered development of hot-dip
Zn--Al--Mg plated steel sheet in the high Al content region.
Formation of this alloy layer markedly degrades plating adhesion,
making use in applications where forming property is important
difficult.
An object of the present invention is therefore to determine the
upper limit of Al content and Mg content in an industrially
producible hot-dip Zn-base plating layer and to provide a high
corrosion resistance hot-dip Zn--Al--Mg plated steel sheet that, in
the high Al content region exceeding 10 mass %, has excellent
quality thoroughly capable of standing up to practical use as an
industrial product.
DISCLOSURE OF THE INVENTION
An in-depth study carried out by the inventors clarified that,
differently from the known corrosion resistance behavior of an
Al-containing Zn-base plated steel sheet, the corrosion resistance
(particularly the outdoor exposure performance) of a hot-dip
Zn--Al--Mg plated steel sheet does not degenerate whatsoever when
the Al content of the plating layer exceeds 10 mass %. This
corrosion resistance behavior, which is not predictable from
conventional knowledge, was concluded to be an effect produced by
combined addition of Al and Mg.
In the hot-dip plating layer Al content region of greater than
around 5 mass %, the melting point of the plating metal rises with
increasing Al content, and the plating bath temperature must be
raised proportionally during the plating operation. However,
increasing the plating bath temperature shortens the service life
of the equipment in the plating bath and tends to increase the
amount of dross in the bath. The higher the Al concentration,
therefore, the more desirable it is to keep the bath temperature as
low as possible, i.e., keep the bath temperature as close to the
melting point as possible. From the viewpoint of obtaining a plated
steel sheet of good appearance when using a Zn--Al--Mg system, it
is important to maintain the metallic structure of the plating
layer in the specified form explained in the following. An
effective way to achieve this is, it was found, to set the plating
bath temperature high, for example, to set a plating bath
temperature that is 40.degree. C. or more higher than the melting
point. Production of a plated steel sheet with good surface
appearance at low cost and high productivity is therefore not easy
in the high plating layer Al content region above 10 mass %.
Further study showed that inclusion of suitable amounts of Ti and B
in the plating layer markedly inhibited generation of the Zn.sub.11
Mg.sub.2 crystal phase that degrades surface appearance. This led
to the discovery that the range of plating bath temperature
conditions within which Zn--Al--Mg plated steel sheet with good
surface appearance is obtainable can be expanded. Moreover, this
effect was found also to be well expressed in the high plating
layer Al content region above 10 mass %. In other words, combined
addition of Ti and B was found to enable production of hot-dip
Zn--Al--Mg plated steel sheet having a plating layer Al content
exceeding 10 mass % at a low plating bath temperature closer to the
melting point of the plating metal.
Moreover, it was ascertained that inclusion of a suitable amount of
Si in the plating layer of a such a high Al hot-dip Zn--Al--Mg
plated steel sheet markedly reduces the amount of alloy layer
generated and, as such, is highly effective for improving plating
adherence. The present invention was accomplished based on the
foregoing newly acquired knowledge.
Specifically, the present invention achieves the foregoing object
by providing a high Al hot-dip Zn--Al--Mg plated steel sheet
obtained by forming on a steel sheet surface a hot-dip plating
layer comprising, in mass %, Al: more than 10 to 22%, Mg: 1-5%, Ti:
0.002-0.1% and B: 0.001-0.045%, and, optionally, Si: 0.005-0.5% and
the balance of Zn and unavoidable impurities.
As a hot-dip Zn--Al--Mg plated steel sheet enabling a good surface
appearance to be obtained with high reliability, the present
invention further provides a high Al hot-dip Zn--Al--Mg plated
steel sheet obtained by forming on a steel sheet surface a hot-dip
Zn-base plating layer of a composition containing, in mass %, Al:
more than 10 to 22% and Mg: 1-5%, which plating layer exhibits a
metallic structure of [primary crystal Al phase] mixed in a matrix
of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure]. In a
preferred aspect, the present invention provides a plated steel
sheet wherein substantially no Zn.sub.11 Mg.sub.2 phase is present
in these metallic structures. By "substantially no Zn.sub.11
Mg.sub.2 phase is present" is meant that the Zn.sub.11 Mg.sub.2
phase is not detected by X-ray diffraction.
The invention further provides plated steel sheets having
preferable compositions of the hot-dip Zn-base plating layer
exhibiting the aforesaid metallic structure. Specifically, the
invention provides as four embodiments plated steel sheets whose
hot-dip Zn-base plating layer composition comprises: i) in mass %,
Al: more than 10 to 22%, Mg: 1-5% and the balance of Zn and
unavoidable impurities, ii) in mass %, Al: more than 10 to 22%, Mg:
1-5%, Ti: 0.002-0.1%, B: 0.001-0.045% and the balance of Zn and
unavoidable impurities, iii) in mass %, Al: more than 10 to 22%,
Mg: 1-5%, Si: 0.005-0.5% and the balance of Zn and unavoidable
impurities, and iv) in mass %, Al: more than 10 to 22%, Mg: 1-5%,
Ti: 0.002-0.1%, B: 0.001-0.045%, Si: 0.005-0.5% and the balance of
Zn and unavoidable impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electron (SEM) micrograph of a plating layer
cross-section in a high Al hot-dip Zn--Al--Mg plated steel sheet in
an example of the present invention, which exhibits a metallic
structure composed of [primary crystal Al phase] mixed in a matrix
of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure].
PREFERRED EMBODIMENTS OF THE INVENTION
In the hot-dip Zn--Al--Mg plated steel sheet of this invention, the
Al in the plating layer mainly serves to improve the corrosion
resistance of the Zn-base plated steel sheet. While conventional
wisdom is that a plating layer Al content in the region of 10-20
mass % tends to degrade rather than improve outdoor exposure
performance, studies conducted by the inventors showed that, to the
contrary, the outdoor exposure performance of a hot-dip Zn--Al--Mg
plated steel sheet does not deteriorate in the high Al region
exceeding 10 mass %. This point will be demonstrated by Examples
set out later in the specification.
As the Al concentration in the Zn-base plating layer increases, the
melting point of the plating metal rises on the side of a higher Al
content than the eutectic composition in the vicinity of Al: about
5 mass % and the heat resistance increases in proportion. In the
region of Al: 10 mass % or less, however, the melting point is low,
i.e., the same as or lower than that pure zinc, so that almost no
effect of heat resistance improvement is obtained, even relative to
ordinary galvanized steel sheet. This invention is therefore
directed to a hot-dip Zn--Al--Mg plated steel sheet whose plating
layer has an Al content exceeding 10 mass %.
When the Al content of the plating layer exceeds 22 mass %, the
melting point becomes 470.degree. C. or higher even when Mg is
present. As the plating bath temperature must therefore be
increased, the service life of equipment immersed in the bath is
shortened and the amount of dross in the bath increases. The
pronounced emergence of these and other operational disadvantages
makes it difficult to provide Zn-base plated steel sheet at a low
cost. This invention therefore defines the upper limit of Al
content in the plating layer as 22 mass %.
Mg in the plating layer produces a uniform corrosion product on the
plating layer surface to markedly enhance the corrosion resistance
of the plated steel sheet. In a Zn-base plated steel sheet whose
plating layer has an Al content exceeding 10 mass %, a marked
corrosion resistance improving effect is observed when the Mg
content of the plating layer is made 1 mass % or greater. When the
Mg is included in excess of 5 mass %, however, the corrosion
resistance improving effect saturates and, disadvantageously, Mg
oxide-system dross generates more readily on the plating bath. The
Mg content of the plating layer is therefore defined as 1-5 mass
%.
When suitable amounts of Ti and B are added to a Zn--Al--Mg hot-dip
plating layer, generation of Zn.sub.11 Mg.sub.2 crystal phase in
the plating layer is markedly inhibited. By taking advantage of
this knowledge, the plating layer of the aforesaid metallic
structure can be formed over a broader range of bath temperature
control than when Ti and B are not added, enabling still more
advantageous and stable production of hot-dip plated steel sheet
that is excellent in corrosion resistance and appearance. Ti and B
are preferably added in combination.
When the Ti content of the plating layer is less than 0.002 mass %,
the effect of inhibition and growth of Zn.sub.11 Mg.sub.2 phase is
not sufficiently manifested. On the other hand, when the Ti content
exceeds 0.1 mass %, Ti--Al-system precipitates occur to produce
"bumps" (known as "butsu" among Japanese field engineers) in the
plating layer that detract from the surface appearance. When Ti is
added, the Ti content of the hot-dip plating is therefore set in
the range of 0.002-0.1 mass %.
When the B content of the hot-melt plating is less than 0.001 mass
%, the effect of inhibiting Zn.sub.11 Mg.sub.2 phase generation and
growth by B is not sufficiently manifested. On the other hand, when
the B content exceeds 0.045 mass %, Al--B-system and Ti--B-system
precipitates occur to produce "bumps" in the plating layer that
detract from the surface appearance. When B is added, the B content
of the hot-dip plating is therefore set in the range of 0.001-0.045
mass %. Within this range of B content, even when a Ti--B-system
compound, e.g., TiB.sub.2, is present in the bath, no "bumps" in
the plating layer because the size of the compound grains is very
small. Therefore, when Ti and B are included in the plating bath,
they can be added as Ti, B or Ti--B alloys, or as Zn alloy, Zn--Al
alloy, Zn--Al--Mg alloy or Al-alloy containing one or more of
these.
Si in the plating layer inhibits generation of an alloy layer
between the steel sheet base metal and the plating layer. In the
high Al hot-dip Zn--Al--Mg plated steel sheet defined by this
invention, the effect of inhibiting the alloy layer is not
sufficient when the Si content of the plating layer is less than
0.005 mass %. On the other hand, when Si is included at a content
exceeding 0.5 mass %, the aforesaid effect saturates and, in
addition, the product quality is degraded by emergence of
Zn--Al--Si--Fe-system dross in the bath. When Si is added to the
plating layer, therefore, its content is preferably controlled to
within the range of 0.005-0.5 mass %.
The metallic structure of the plating layer will now be
explained.
As explained in the foregoing, it was found that when a high Al
hot-dip Zn--Al--Mg plated steel sheet is produced by forming on the
surface of a steel sheet a hot-dip Zn-base plating layer of a
composition containing, in mass %, Al: more than 10 to 22% and Mg:
1-5%, its surface appearance and corrosion resistance are degraded
when crystallization of Zn.sub.11 Mg.sub.2 occurs. In contrast, a
high Al hot-dip Zn--Al--Mg plated steel sheet whose plating layer
structure is a metallic structure of [primary crystal Al phase]
mixed in a matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal
structure] is excellent in appearance and also very good in
corrosion resistance.
In the metallic structure of [primary crystal Al phase] mixed in a
matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure],
the total amount of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal
structure]+[primary crystal Al phase] is preferably 80 vol. % or
greater, more preferably 95 vol. % or greater. The balance can be a
mixture of small amounts of Zn single phase, [Zn/Zn.sub.2 Mg]
binary eutectic crystal, Zn.sub.2 Mg phase and [Al/Zn.sub.2 Mg]
binary eutectic crystal. When Si is added, small amounts of Si
phase, Mg.sub.2 Si phase and [Al/Mg.sub.2 Si] binary eutectic
crystal may also be mixed therein.
FIG. 1 is an electron (SEM) micrograph showing an example of a
plating layer cross-section exhibiting a metallic structure
composed of [primary crystal Al phase] mixed in a matrix of
[Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure]. The plating
layer of this micrograph is a Ti-- and B-added material having a
basic composition of Zn--15 mass % Al--3 mass % Mg. The blackish
portion at the bottom of the micrograph is the steel sheet base
metal. In the metallic structure of the plating layer present on
the steel sheet base metal, the eutectic composition of the matrix
is the [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure] and
the large, blackish island-like portions are the [primary crystal
Al phase]. No Zn.sub.11 Mg.sub.2 phase was be observed in the
metallic structure by X-ray diffraction.
In the high Al hot-dip Zn--Al--Mg plated steel sheet exhibiting the
metallic structure described in the foregoing, the invention
defines the plating layer to be a hot-dip Zn-base plating layer of
a composition containing, in mass %, Al: more than 10 to 22% and
Mg: 1-5%. Although this hot-dip Zn-base plating layer is required
to contain 50 mass % or more of Zn, it may, in addition to Al, Mg
and Zn, also contain other elements to an extent that does not
degrade the basic characteristics of the plated steel sheet that
the invention aims to achieve, specifically the corrosion
resistance and surface appearance.
For example, the hot-dip Zn-base plating layer may be one
containing Ti and B for inhibiting generation of Zn.sub.11 Mg.sub.2
phase, one containing Si for inhibiting alloy layer formation, one
containing Ni (which is thought to have an effect of improving
corrosion resistance at worked portions) at a content of, for
example, 0.1-1 mass %, one containing, for example, 0.001-1.0 mass
% of Sr for stabilizing the properties of an oxide coating of the
plating layer surface to thereby inhibit "wrinkle-like surface
defects," one containing one or more of Na, Li, Ca and Ba (which
are thought to have a similar effect) at, for example, a total of
0.01-0.5 mass %, one containing rare earth elements (which are
thought to improve plating property and inhibit plating defects)
at, for example, a total of 0.0005-1 mass %, one containing Co
(which is thought to improve the luster-retention property of the
plating surface) at, for example, 0.01-1 mass %, and one containing
Sb and Bi (which are thought to improve the intergranular corrosion
resistance of the plating layer) at, for example, a total of
0.005-0.5 mass %.
As regards the specific hot-dip Zn-base plating layer, the
invention defines the following four composition types: i) one
comprising, in mass %, Al: more than 10 to 22%, Mg: 1-5% and, the
balance of Zn and unavoidable impurities, ii) one comprising, in
mass %, Al: more than 10 to 22%, Mg: 1-5%, Ti: 0.002-0.1%, B:
0.001-0.045% and the balance of Zn and unavoidable impurities, iii)
one comprising, in mass %, Al: more than 10 to 22%, Mg: 1-5%, Si:
0.005-0.5% and the balance of Zn and unavoidable impurities, and
iv) one comprising, in mass %, Al: more than 10 to 22%, Mg: 1-5%,
Ti: 0.002-0.1%, B: 0.001-0.045%, Si: 0.005-0.5% and the balance of
Zn and unavoidable impurities.
These four compositions may, as impurity, include Fe up to about 1
mass which is the Fe content ordinarily allowed in a hot-dip
Zn-base plating bath.
The coating weight of the plating is preferably adjusted to 25-300
g/m.sup.2 per side of the steel sheet. A plating bath temperature
exceeding 550.degree. C. is undesirable because evaporation of zinc
from the bath becomes pronounced, making plating defects likely to
occur, and the amount of oxide dross on the bath surface
increases.
EXAMPLES
Example 1
Hot-dip Zn--Al--Mg plated steel sheets (containing no added T, B or
Si) were produced to have various Al and Mg contents using a
continuous hot-dip plating simulator (continuous hot-dip plating
test line). The plating conditions were as set out below.
Plating Conditions
Processed Steel Sheet:
Cold-rolled, low-carbon, Al-killed steel (Thickness: 0.8 mm)
Running Speed:
100 m/min
Plating Bath Composition (Mass %):
As shown in Table 1
Plating Bath Temperature:
When Al=10.8%: 470.degree. C.
When Al=15.2%: 485.degree. C.
When Al=21.7%: 505.degree. C.
Plating Bath Immersion Time:
2 sec
Wiping Gas:
Air
Coating Weight (Per Side):
60 g/m.sup.2
Mean Cooling Rate from Bath Temperature to Plating Layer
Solidification Temperature:
4.degree. C./sec
The occurrence of dross in the bath was visually observed during
plating with each plating bath and was compared with that in the
manufacture of ordinary hot-dip galvanized steel sheet. A bath in
which the amount of dross generated was low and about equal to the
ordinary level was rated good and assigned the symbol {character
pullout}, one that generated a somewhat large amount that was
liable to have an adverse effect on the plated steel sheet quality
was rated fair and assigned the symbol A, and one that generated a
large amount that clearly degraded the quality of the steel sheet
and also impeded continuous operation was rated poor and assigned
the symbol X. Further, the steel sheets obtained were subjected to
a 24-month outdoor exposure test at a seaside industrial area in
Sakai City, Japan and the amount of corrosion loss was measured.
The results are shown in Table 1.
Although not indicated in Table 1, the metallic structure of the
plating layer of each sample was determined to consist of [primary
crystal Al phase] mixed in a matrix of [Al/Zn/Zn.sub.2 Mg ternary
eutectic crystal structure]. All of the steel sheets were good in
appearance but some were found to include small amounts of Zn
single phase, Zn/Zn.sub.2 Mg binary eutectic crystal, Al/Zn.sub.2
/Mg binary eutectic crystal, Zn.sub.2 Mg phase and the like.
Invention Examples No. A3-A5, A9-A11 and A15-A17 were examined by
X-ray diffraction. Presence of Zn.sub.11 Mg.sub.2 phase was not
observed.
TABLE 1 Plating layer composition (balance Zn) Corrosion Dross
(mass %) loss generation Example No. Al Mg Ti B Si (g/m.sup.2)
rating type A1 10.8 0 0 0 0 8.5 .circleincircle. Comparative A2
10.8 0.5 0 0 0 8.1 .circleincircle. Comparative A3 10.8 1.2 0 0 0
4.3 .circleincircle. Invention A4 10.8 3.1 0 0 0 4.2
.circleincircle. Invention A5 10.8 4.2 0 0 0 4.2 .circleincircle.
Invention A6 10.8 5.5 0 0 0 4.2 .DELTA. Comparative A7 15.2 0 0 0 0
10.6 .circleincircle. Comparative A8 15.2 0.5 0 0 0 10.1
.circleincircle. Comparative A9 15.2 1.2 0 0 0 4.4 .circleincircle.
Invention A10 15.2 3.1 0 0 0 4.2 .circleincircle. Invention A11
15.2 4.8 0 0 0 4.2 .circleincircle. Invention A12 15.2 6.1 0 0 0
4.2 X Comparative A13 21.7 0 0 0 0 13.1 .circleincircle.
Comparative A14 21.7 0.5 0 0 0 12.8 .circleincircle. Comparative
A15 21.7 1.2 0 0 0 4.5 .circleincircle. Invention A16 21.7 3.1 0 0
0 4.3 .circleincircle. Invention A17 21.7 4.8 0 0 0 4.3
.circleincircle. Invention A18 21.7 5.8 0 0 0 4.3 X Comparative
Example 2
Hot-dip Zn--Al--Mg plated steel sheets (containing added Ti and B;
no added Si) were produced to have various Al and Mg contents using
a continuous hot-dip plating simulator (continuous hot-dip plating
test line). The plating conditions were as set out below.
Plating Conditions
Processed Steel Sheet:
Hot-rolled, medium-carbon, Al-killed steel (Thickness: 2.3 mm)
Running Speed:
40 m/min
Plating Bath Composition (Mass %):
As shown in Table 2
Plating Bath Temperature:
When Al=10.5%: 445.degree. C.
When Al=13.9%: 480.degree. C.
When Al=21.1%: 500.degree. C.
Plating Bath Immersion Time:
5 sec
Wiping Gas:
Nitrogen (Oxygen concentration: less than 1%)
Coating Weight (Per Side):
200 g/m.sup.2
Mean Cooling Rate from Bath Temperature to Plating Layer
Solidification Temperature:
4.degree. C./sec
Occurrence of dross in the bath was evaluated and corrosion loss
was investigated by conducting an outdoor exposure test. The
methods used were the same as those in Example 1. The results are
shown in Table 2.
The metallic structure of the plating layer of each sample was
determined to consist of [primary crystal Al phase] mixed in a
matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure].
All of the steel sheets were good in appearance but some were found
to include small amounts of Zn single phase, Zn/Zn.sub.2 Mg binary
eutectic crystal, Al/Zn.sub.2 /Mg binary eutectic crystal, Zn.sub.2
Mg phase and the like. Invention Examples No. B3-B6, B9-B11 and
B15-B17 were examined by X-ray diffraction. Presence of Zn.sub.11
Mg.sub.2 phase was not observed.
TABLE 2 Plating layer composition Dross (balance Zn) Corrosion
genera- (mass %) loss tion Example No. Al Mg Ti B Si (g/m.sup.2)
rating type B1 10.5 0 0.03 0.006 0 8.5 .circleincircle. Comparative
B2 10.5 0.5 0.03 0.006 0 8.2 .circleincircle. Comparative B3 10.5
1.2 0.03 0.006 0 4.3 .circleincircle. Invention B4 10.5 2.1 0.03
0.006 0 4.2 .circleincircle. Invention B5 10.5 3.1 0.03 0.006 0 4.2
.circleincircle. Invention B6 10.5 4.1 0.03 0.006 0 4.2
.circleincircle. Invention B7 13.9 0 0.03 0.006 0 10.1
.circleincircle. Comparative B8 13.9 0.5 0.03 0.006 0 9.6
.circleincircle. Comparative B9 13.9 1.2 0.03 0.006 0 4.3
.circleincircle. Invention B10 13.9 3.1 0.03 0.006 0 4.1
.circleincircle. Invention B11 13.9 4.8 0.03 0.006 0 4.2
.circleincircle. Invention B12 13.9 6.1 0.03 0.006 0 4.2 X
Comparative B13 21.1 0 0.03 0.006 0 13.2 .circleincircle.
Comparative B14 21.1 0.5 0.03 0.006 0 12.1 .circleincircle.
Comparative B15 21.1 1.2 0.03 0.006 0 4.5 .circleincircle.
Invention B16 21.1 3.1 0.03 0.006 0 4.3 .circleincircle. Invention
B17 21.1 4.8 0.03 0.006 0 4.3 .circleincircle. Invention B18 21.1
5.8 0.03 0.006 0 4.3 X Comparative
Example 3
Hot-dip Zn--Al--Mg plated steel sheets (containing no added Ti and
B; containing added Si) were produced to have various Al and Mg
contents using a continuous hot-dip plating simulator (continuous
hot-dip plating test line). The plating conditions were as set out
below.
Plating Conditions
Processed Steel Sheet:
Cold-rolled, very low-carbon, Ti-added, Al-killed steel (Thickness:
0.8 mm)
Running Speed:
100 m/min
Plating Bath Composition (Mass %):
As shown in Table 3
Plating Bath Temperature:
When Al=10.8%: 470.degree. C.
When Al=15.2%: 485.degree. C.
When Al=21.7%: 505.degree. C.
Plating Bath Immersion Time:
2 sec
Wiping Gas:
Nitrogen (Oxygen concentration: less than 1%)
Coating Weight (Per Side):
100 g/m.sup.2
Mean Cooling Rate from Bath Temperature to Plating Layer
Solidification Temperature:
4.degree. C./sec
Occurrence of dross in the bath was evaluated and corrosion loss
was investigated by conducting an outdoor exposure test. The
methods used were the same as those in Example 1. The results are
shown in Table 3.
The metallic structure of the plating layer of each sample was
determined to consist of [primary crystal Al phase] mixed in a
matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure].
All of the steel sheets were good in appearance but some were found
to include small amounts of Zn single phase, Zn/Zn.sub.2 Mg binary
eutectic crystal, Al/Zn.sub.2 /Mg binary eutectic crystal, Zn.sub.2
Mg phase, Si phase, Mg.sub.2 Si phase, Al/Mg.sub.2 Si binary
eutectic crystal and the like. Invention Examples No. C3-C5,
C9-C.sub.11 and C15-C17 were examined by X-ray diffraction.
Presence of Zn.sub.11 Mg.sub.2 phase was not observed.
TABLE 3 Plating layer composition (balance Zn) Corrosion Dross
(mass %) loss generation Example No. Al Mg Ti B Si (g/m.sup.2)
rating type C1 10.8 0 0 0 0.02 8.4 .circleincircle. Comparative C2
10.8 0.5 0 0 0.02 8.1 .circleincircle. Comparative C3 10.8 1.2 0 0
0.02 4.4 .circleincircle. Invention C4 10.8 3.1 0 0 0.02 4.2
.circleincircle. Invention C5 10.8 4.2 0 0 0.02 4.2
.circleincircle. Invention C6 10.8 5.5 0 0 0.02 4.2 .DELTA.
Comparative C7 15.2 0 0 0 0.02 10.7 .circleincircle. Comparative C8
15.2 0.5 0 0 0.02 10.2 .circleincircle. Comparative C9 15.2 1.2 0 0
0.02 4.3 .circleincircle. Invention C10 15.2 3.1 0 0 0.02 4.2
.circleincircle. Invention C11 15.2 4.8 0 0 0.02 4.2
.circleincircle. Invention C12 15.2 6.1 0 0 0.02 4.2 X Comparative
C13 21.7 0 0 0 0.02 13.1 .circleincircle. Comparative C14 21.7 0.5
0 0 0.02 12.6 .circleincircle. Comparative C15 21.7 1.2 0 0 0.02
4.4 .circleincircle. Invention C16 21.7 3.1 0 0 0.02 4.3
.circleincircle. Invention C17 21.7 4.8 0 0 0.02 4.3
.circleincircle. Invention C18 21.7 5.8 0 0 0.02 4.3 X
Comparative
Example 4
Hot-dip Zn--Al--Mg plated steel sheets (containing added Ti, B and
Si) were produced to have various Al and Mg contents using a
continuous hot-dip plating simulator (continuous hot-dip plating
test line). The plating conditions were as set out below.
Plating Conditions
Processed Steel Sheet:
Hot-rolled, low-carbon, Al-killed steel (Thickness: 2.3 mm)
Running Speed:
40 m/min
Plating Bath Composition (Mass %):
As shown in Table 4
Plating Bath Temperature:
When Al=10.5%: 445.degree. C.
When Al=13.5%: 480.degree. C.
When Al=20.1%: 500.degree. C.
Plating Bath Immersion Time:
5 sec
Wiping Gas:
Nitrogen (Oxygen concentration: less than 2%)
Coating Weight (Per Side):
150 g/m.sup.2
Mean Cooling Rate from Bath Temperature to Plating Layer
Solidification Temperature:
4.degree. C./sec
Occurrence of dross in the bath was evaluated and corrosion loss
was investigated by conducting an outdoor exposure test. The
methods used were the same as those in Example 1. The results are
shown in Table 4.
The metallic structure of the plating layer of each sample was
determined to consist of [primary crystal Al phase] mixed in a
matrix of [Al/Zn/Zn.sub.2 Mg ternary eutectic crystal structure].
All of the steel sheets were good in appearance but some were found
to include small amounts of Zn single phase, Zn/Zn.sub.2 Mg binary
eutectic crystal, Al/Zn.sub.2 /Mg binary eutectic crystal, Si
phase, Mg.sub.2 Si phase, Al/Mg.sub.2 Si binary eutectic crystal
and the like. Invention Examples No. D3-D6, D9-D11 and D15-D17 were
examined by X-ray diffraction. Presence of Zn.sub.11 Mg.sub.2 phase
was not observed.
TABLE 4 Plating layer composition Corro- Dross (balance Zn) sion
genera- (mass %) loss tion Example No. Al Mg Ti B Si (g/m.sup.2)
rating type D1 10.5 0 0.03 0.006 0.05 8.5 .circleincircle.
Comparative D2 10.5 0.5 0.03 0.006 0.05 8.2 .circleincircle.
Comparative D3 10.5 1.2 0.03 0.006 0.05 4.4 .circleincircle.
Invention D4 10.5 2.1 0.03 0.006 0.05 4.2 .circleincircle.
Invention D5 10.5 3.1 0.03 0.006 0.05 4.2 .circleincircle.
Invention D6 10.5 4.1 0.03 0.006 0.05 4.2 .circleincircle.
Invention D7 13.5 0 0.03 0.006 0.05 10.5 .circleincircle.
Comparative D8 13.5 0.5 0.03 0.006 0.05 9.9 .circleincircle.
Comparative D9 13.5 1.2 0.03 0.006 0.05 4.3 .circleincircle.
Invention D10 13.5 3.1 0.03 0.006 0.05 4.2 .circleincircle.
Invention D11 13.5 4.8 0.03 0.006 0.05 4.2 .circleincircle.
Invention D12 13.5 6.1 0.03 0.006 0.05 4.2 X Comparative D13 20.1 0
0.03 0.006 0.05 13.5 .circleincircle. Comparative D14 20.1 0.5 0.03
0.006 0.05 12.5 .circleincircle. Comparative D15 20.1 1.2 0.03
0.006 0.05 4.4 .circleincircle. Invention D16 20.1 3.1 0.03 0.006
0.05 4.3 .circleincircle. Invention D17 20.1 4.8 0.03 0.006 0.05
4.3 .circleincircle. Invention D18 20.1 5.8 0.03 0.006 0.05 4.3 X
Comparative
Example 5
Hot-dip Zn--Al--Mg plated steel sheets (containing no added Ti or
B) were produced to have various Si contents using a continuous
hot-dip plating simulator (continuous hot-dip plating test line).
The plating bath had a basic composition of Zn--15.0 mass % Al--3.0
mass % Mg. The plating conditions were as set out below.
Plating Conditions
Processed Steel Sheet:
Cold-rolled, low-carbon, Al-killed steel (Thickness: 0.8 mm)
Running Speed:
100 m/min
Plating Bath Composition (Mass %):
Zn--15.0 mass % Al--3.0 mass % Mg--.dagger.Si (.dagger.: As shown
in Table 5)
Plating Bath Temperature:
470.degree. C.
Plating Bath Immersion Time:
3 sec
Wiping Gas:
Air
Coating Weight (Per Side):
250 g/m.sup.2
Mean Cooling Rate from Bath Temperature to Plating Layer
Solidification Temperature:
7.degree. C./sec
Occurrence of dross in the bath was evaluated and corrosion loss
was investigated by conducting an outdoor exposure test. The
methods used were the same as those in Example 1. The results are
shown in Table 4.
The mean thickness of the alloy layer of each sample was determined
by observing the metallic structure of a plating layer
cross-section with an electron microscope (SEM). The results are
shown in Table 5. The mean alloy layer thickness of samples whose
plating layer had an Si content of 0.05 mass % or greater was less
than 0.1 .mu.m. These samples exhibited high plating adherence and
were more than adequate for applications involving heavy working.
In the case of Si content of 0.7 mass %, a large amount of
Zn--Al--Si--Fe-system dross was generated.
TABLE 5 Si content of plating layer (.dagger.) Mean thickness of
alloy layer (mass %) (.mu.m) 0 5 0.003 3 0.005 0.5 0.01 0.2 0.05
Less than 0.1 0.1 Less than 0.1 0.5 Less than 0.1 0.7 Less than
0.1
As shown above, the research carried out by the inventors clarified
that the outdoor exposure performance of high Al hot-dip Zn--Al--Mg
plated steel sheet does not degenerate in the high plating layer Al
content region above 10 mass %. It also identified a metallic
structure that enables good surface appearance to be obtained with
high reliability in such a high Al hot-dip Zn--Al--Mg plated steel
sheet. The inventors also ascertained that inclusion of suitable
amounts of Ti and B in the plating layer facilitates the hot-dip
plating operation by lowering the plating bath temperature and that
inclusion of a suitable amount of Si suppresses the amount of alloy
layer to ensure good plating adherence. As a result, the adverse
effects that increasing the Al content of a Zn--Al--Mg plated steel
sheet to a high level has on the plating operation and the quality
of the product can be considerably reduced. The present invention
therefore makes a major contribution to industrial utilization of
high Al hot-dip Zn--Al--Mg plated steel sheet, which has heretofore
been considered hard to commercialize.
* * * * *